The world doesn’t use TAI. If I’m making a train booking app and the train departs at 21:50 in Germany in the summer, it’s not departing at 21:50 TAI+2, it’s departing at 21:50 UTC+2. So you cannot avoid using UTC.
Programmers simply deal with the world as it exists. Every single country uses UTC therefore that is how time must be represented. It is easier to remove leap seconds from UTC than it is to make all countries adopt TAI at the same instant.
The concept of leap seconds can of course be preserved by defining a new time standard that isn’t called “UTC”.
TAI is ‘a second always takes the same amount of time, and the number of seconds is always increasing’. Which is really useful in the real world when tracking/counting events every second all the time.
Because leap seconds cause weird gaps or overlaps (depending on how they are happening).
This doesn’t matter for pretty much anyone who doesn’t track/log/act every second all the time in a way where if a second ‘disappears’ or happens twice or takes longer on one day than on another it’s noticeable.
When monitoring or tracking large scale systems, it’s a really irritating problem, which is why TAI is nice and exists in computer land. Also why it’s nice for many scientist types.
For everyone else, something like UTC layered on top is more than good enough. Leap seconds are fine enough there. Same with most timezones, and PDT/PST, etc.
Programmers simply deal with the world as it exists. Every single country uses UTC therefore that is how time must be represented. It is easier to remove leap seconds from UTC than it is to make all countries adopt TAI at the same instant.
The concept of leap seconds can of course be preserved by defining a new time standard that isn’t called “UTC”.